Histoplasma capsulatum grows and reproduces as a saprobe in guano-enriched soils, and from these histoplasmosis may be contracted by humans and other mammals. Variations will be determined in isolates by means of isozyme phenotypes obtained from selected soil populations with different bird and bat excreta, and studies will be done to compare their population structures and levels of phenotypic variability within and among populations. The genetic basis for this variability will be studied. Comparisons will be made with isolates obtained from clinical specimens. Mehtods of directly isolating the fungus (without passage through mice) will be sought, and the general levels of virulence of such soil isolates determined. A technique for concentrating conidia and mycelial fragments by filtration from soils will be used together with fluorescent antibodies to locate microhabitats and to compare soils quantitatively for presence of the fungus.